1. Field of the Invention
The present invention relates to a control system for a hybrid vehicle that is driven by an internal combustion engine and an electric motor, and is capable of recovering electric energy for driving the electric motor, using the output from the engine.
2. Description of the Related Art
Conventionally, a control system of this kind has been proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2002-242718. This control system is configured such that the vehicle is driven by the engine during normal operation of the vehicle, such as cruising, and when the degree of increase in the throttle valve opening is large, the vehicle is driven not only by the engine but also by the electric motor n an auxiliary manner so as to ensure the feeling of acceleration. Further, depending on the operating conditions of the engine, the electric motor operates as a generator to generate electric energy using the output from the engine to thereby charge a battery as the drive source of the electric motor with the generated electric energy.
This kind of hybrid vehicle aims to attain excellent fuel economy as one of its goals, and to attain this goal, it is necessary to operate the engine while properly selecting between an engine-driven mode in which the vehicle is driven by the engine and a motor-driven mode in which the vehicle is driven by the electric motor. However, the magnitude relationship between the fuel consumption in the engine-driven mode and that in the motor-driven mode during which recovery of electric energy is additionally carried out is not definite but varies with the operating conditions of the engine. To comply with this varying relationship between the fuel consumption in the engine-driven mode and that in the motor-driven mode, the proposed control system only selects the use of the engine-driven mode during the normal operation, such as cruising, and the combined use of the engine-driven mode and the motor-driven mode e.g. during acceleration. This roughly selective use of the two modes cannot attain desired excellent fuel economy.